Build this with RelaxNG <include> feature instead of a complete schema.

This isn't bad but could work better if the schema were a little more
modular...

In order to track down all the uses of extension namespaces like loext,
invoke the validator without "-e".

Change-Id: I92a666fc54c25d66cea38decdc188d60de2a0b74
Reviewed-on: https://gerrit.libreoffice.org/56488
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>